The present invention generally relates to micromachining devices and processes for their fabrication. More particularly, this invention relates to a microfluidic device having a compact micromachined freestanding member configured to sense one or more properties of a fluid flowing through an internal passage within the freestanding member.
FIGS. 1 and 2 represent a Coriolis-based fluid sensing device 10 of a type disclosed in commonly-assigned U.S. Pat. No. 6,477,901 to Tadigadapa et al., whose contents relating to the fabrication and operation of a Coriolis-based sensor are incorporated herein by reference. The fluid sensing device 10 is represented as including a substrate 12 that may be formed of silicon or another semiconductor material, quartz, glass, ceramic, metal, a polymeric material, a composite material, etc. A tube 14 is supported by the substrate 12 so as to have a base 28 attached to a surface 18 of the substrate 12 and a freestanding portion 16 suspended above the substrate 12. As evident from FIG. 1, the freestanding portion 16 has a generally U or D-shaped configuration. Electrodes 22 and 24 are located on the substrate 12 beneath the freestanding portion 16 of the tube 14, and bond pads 32 (only one of which is shown) are provided for transmitting input and output signals to and from the device 10. The electrode 22 can be, for example, capacitively coupled to the tube 14 for capacitively (electrostatically) driving the freestanding portion 16 at or near resonance, while the remaining electrodes 24 sense (e.g., capacitively) the deflection of the tube 14 relative to the substrate 12 and provide feedback to enable the vibration frequency induced by the drive electrode 22 to be controlled with appropriate circuitry. With a fluid entering the device 10 through an inlet port 26 and flowing through an internal passage 20 within the tube 14, the freestanding portion 16 can be vibrated at or near resonance to ascertain certain properties of the fluid, such as flow rate and density, using Coriolis force principles. Notable advantages of the device 10 include the extremely miniaturized scale to which it can be fabricated and its ability to precisely analyze very small quantities of fluids. In FIG. 2, the device 10 is schematically shown as enclosed by a cap 30 to allow for vacuum packaging that further improves the performance of the device 10 by reducing air damping effects.
Tadigadapa et al., commonly-assigned U.S. Pat. No. 6,647,778 to Sparks, and commonly assigned U.S. Patent Application Publication No. 2006/0175303 to Sparks et al. disclose processes for fabricating flow sensing devices of the type shown in FIGS. 1 and 2 using micromachining techniques. As used herein, micromachining is a technique for forming very small elements by bulk etching a substrate (e.g., a silicon wafer), and/or by surface thin-film etching, the latter of which generally involves depositing a thin film (e.g., polysilicon or metal) on a sacrificial layer (e.g., oxide layer) on a substrate surface and then selectively removing portions of the sacrificial layer to free the deposited thin film. As disclosed by Tadigadapa et al., Sparks, and Sparks et al., wafer bonding and silicon etching techniques can be used to produce microelectromechanical systems (MEMS) comprising one or more flow sensing devices. Sensors of the type taught by Tadigadapa et al. have found use in a variety of applications, as evident from Sparks, Sparks et al., commonly-assigned U.S. Pat. Nos. 6,932,114, 6,942,169, and 7,059,176, and U.S. Patent Application Publication Nos. 2004/0171983, 2005/0126304, 2005/0235759, 2005/0284815, 2006/0010964, and 2006/0213552. As examples, the teachings of Tadigadapa et al. have been applied to mass flow sensors, density sensors, fuel cell concentration meters, chemical concentration sensors, specific gravity sensors, pressure sensors, temperature sensors, drug infusion devices, and other devices that can employ resonating and stationary microtubes. Nonetheless, further improvements would be desirable for use in the design and fabrication of devices such as Tadigadapa et al. that employ extremely miniaturized fluid channels, including the capability of further reducing the size of such devices.